The word Kilometer is a combination of the prefix "kilo" (meaning thousand) with the word meter. That is the reason why one kilometer equals exactly one thousand meters. In spite of this, it should be noted that the abbreviation for the word kilometer is "km". To finish, never place the plural in the kilometer symbol, that is, whether it is 1 km or 8 km, the abbreviation should always appear in the singular.
The first time the term Mile was used to denote distance was in Ancient Rome. In this case, the earth mile was equivalent to 1000 steps (from the Latin, mille passus) given by the Centurion. Obviously, this measure was not accurate, since it depended on the size of the centurion's legs. In the case of the sea or nautical mile that came later, its calculation is much more scientific. Knowing that the equator line measures approximately 40,000 km, this value was divided by the 360º that this line does when circumventing the planet and then by 60 to convert to minutes. Thus a correspondence was established between the measurement of the equator and the sea mile.